Hydrocarbon feeds derived from petroleum are commonly cracked to produce a product containing lower molecular weight hydrocarbons for use for various purposes. The cracked products generally contain olefins which are useful reactants for various purposes. Among olefins which may be present in cracked products are tertiary olefins, for example C.sub.4 (iso-butene), C.sub.5 and higher tertiary olefins. Iso-butene may be reacted with alkanols to give alkyl tertiary butyl ethers. Thus iso-butene may be reacted with methanol to give MTBE (methyl tertiary butyl ether). The C.sub.5 and higher tertiary olefins may be reacted with alkanols to give alkyl tertiary alkyl ethers. Thus C.sub.5 tertiary olefins may be reacted with methanol to give TAME (tertiary amyl methyl ether). These ethers are well-known to be useful as additives for gasoline.
Petroleum contains various sulphur and nitrogen compounds which can have an adverse affect on the activity of catalysts. In particular fractions obtained by cracking high boiling petroleum fractions can contain undesirable impurities. The etherification reactions mentioned above are generally carried out using acidic catalysts, for example acidic ion exchange resins. It has been found that basic nitrogen compounds present in cracked products used as feed to such etherification reactions can have an adverse effect on the activity of the acidic catalyst. Guard beds containing an acidic material, such as the acidic ion exchange resin, have been used to remove basic compounds before the olefin feed is reacted with the alkanol.
We have now found that despite the use of such guard beds the activity of catalysts used to react C.sub.5 tertiary olefins with alkanols has declined due to the presence of one or more catalyst poisons which appear able to pass through the conventional guard beds. We have now found a new procedure for lowering the content of an undesirable impurity from fractions containing C.sub.5 or C.sub.6 hydrocarbons obtained by cracking materials derived from petroleum.